Salt -- sodium chloride -- constitutes about 0.3 percent of the satellite's atmosphere, along with sulphur dioxide and sulphur monoxide, two previously-spotted gases that are classic byproducts of high-temperature eruptions, they say in a study published in Nature, the British science weekly.

The salt could help explain mysterious clouds of sodium and chlorine that have escaped Io's gravity and are now orbiting the mother planet, they added.

The research was led by Emmanuel Lellouch of the Paris-Meudon observatory.

In November, astronomers at the Keck II observatory reported having seen a huge eruption on Io that was many times more powerful than any volcanic activity during human history on Earth.

Io, with a diameter of 3,640 kilometers (2,275 miles), is one of the four large "Galilean" moons reputedly first spotted by the Italian astronomer Galileo in 1610. The others are Europa, Callisto and Ganymede.

SPACE.WIRE